Microfluidic devices are having an increasing impact on biomedical diagnostics and drug development. Centripetal (or centrifugal) microfluidics in particular can deal with very small volumes of liquid, usually in the microliter range. At such a small scale, the surface to volume ratio of flowing liquids increases drastically, while at the same time the specific Reynolds number becomes very small (typically less than 1). In such Reynolds number regimes the flow is always laminar, turbulences of any kind being completely forbidden. This is a serious drawback for mixing two or more liquids since, in the absence of any turbulence, diffusive mixing is the only available mechanism. This is an inherently very slow process. At the microfluidic scale, diffusion lengths of at least hundreds of microns are necessary, and for diffusion constants on the order of 10−12 (e.g. for DNA molecules (Robertson 2006)) corresponding diffusion times of about several tens of minutes are unavoidable. Consequently, the aim of any microfluidic mixing scheme is to enhance performance of the mixing process and achieve certain mixing performance within a minimum footprint and time.
There are few recent and good review articles for the state of the art in microfluidic mixing (Suh 2010; Capretto 2011; Lee 2011). As pointed out by these articles, while classical continuous microfluidics has been the field of several advancements and innovations in this matter, there is much on-going research and unsolved problems in centrifugal microfluidic mixing field. For the most part, mixing applications are designed by directly transferring knowledge from traditional microfluidic mixing to centrifugal microfluidics. In a recent example (Grumann 2005), a centrifugal microfluidic platform is achieved by magnetically stirring beads in a mixing chamber or by generating inertia effects trough sudden accelerations and decelerations of the platform. In another example (Noroozi 2009), Coriolis force and alternate spinning is used for the same purpose. In a relatively different approach, mixing in the channels can be achieved by generating vortices through appropriate twisting the flow with various ridge- and herringbone-structures (Stroock 2002). However, in principle, any other method of mixing used in traditional microfluidics either active (acoustic, ultrasonic, dielectrophoretic, electrodynamic, electrokinetic, etc.) or passive (lamination, zigzagging, 3D serpentines, etc.) can be used. The price to pay for this simple transfer of technology from traditional to centrifugal microfluidics is the complexity of both actuation and handling and the final fabrication cost per unit device. These approaches are generally not appropriate for centrifugal microfluidic devices and do not take advantage of several features these platforms may offer.
In recent work (Coleman 2006), a sequential (active) injection of liquids in the same channel followed by an expansion chamber for enhancing diffusive mixing may be employed. Similarly, the ability to generate very small liquid droplets by simply terminating microfluidic channels with a large reservoir (chamber) and putatively alternating liquid layers may be employed (Burger 2009). However, this approach results in inefficient mixing. As a consequence, the mixing time obtained is too long.
There remains a need for efficient mixing of fluids in a centrifugal microfluidic device.